Thin polished plates such as silicon wafers and the like are a very important part of modern technology. A wafer, for instance, may refer to a thin slice of semiconductor material used in the fabrication of integrated circuits and other devices. Other examples of thin polished plates may include magnetic disc substrates, gauge blocks and the like. While the technique described here refers mainly to wafers, it is to be understood that the technique also is applicable to other types of polished plates as well. The term wafer and the term thin polished plate may be used interchangeably in the present disclosure.
Generally, certain requirements may be established for the flatness and thickness uniformity of the wafers. There exist a variety of techniques to address the measurement of shape and thickness variation of wafers. However, most existing wafer inspection tools can only inspect the wafer surface and detect the surface defects. Some wafer surface inspection tools may also calculate the defect areas and classify the defect types, but there is no information reported about the defect height or depth by the existing inspection tools.